// second.rs
pub fn message() -> String {
    String::from("This is the 2nd module.")
}

pub mod car {
    pub mod bicycle {
        pub fn run() -> String {
            String::from("This is a bicycle.")
        }
    }
    pub mod bus {
        pub fn run() -> String {
            String::from("This is a bus.")
        }
    }

    // use 关键字可以与 pub 关键字配合使用：
    pub use bicycle::run as bic_run;
    pub use bus::run as bu_run;
}